1. Field of the Invention
The invention relates in general to a method and apparatus for preparing the reproduction of a document based on scanning the original, and in particular, to such a method and apparatus utilizing the output of a dither circuit to represent grey toned values.
2. Description of the Prior Art
In general, when optical scanners scan, they generate an electronic signal representing grey tone values for points along lines across the scanned document. The grey tone values are typically represented as a digital multi-bit signal. Thereby the threshold value may be adapted to the foreground/background variations in shades of grey across the document to remove "noise" from the scanned data. Thresholding is used to convert the multi-bit, e.g. an 8-bit representation of a pixel, to a 1-bit representation.
Using this conversion white and pale grey areas become white, and black and dark grey areas become black. The shades represented by the grey tones in between black and white will be lost. This is a disadvantage if the document has to be reproduced on an output device for viewing, as information communicated in the grey tones of the document is lost. An advantage is that because the "noise" is removed by the adaptive threshold process, the 1-bit representation may be compacted, thereby reducing the amount of data to be transferred for reproduction or to be stored on a storage device.
Alternatively, a 1-bit representation can be achieved by halftoning, whereby grey areas keep their overall grey appearance due to a dither distribution of black and white pixels in the grey areas corresponding to the original grey tones on the scanned document. A disadvantage is that because "noise" is added by the halftoning process, the 1-bit representation cannot be very compacted, thereby increasing the amount of data to be transferred for reproduction or to be stored on a storage device. Another disadvantage is that dark grey areas representative of foreground features are dithered to a grey appearance instead of black, thereby reducing the contrast of the reproduction.
Halftoning is described in "A Fast Algorithm for General Raster Rotation", from Graphics Interface '86, by a Paeth (1986); "On the Error diffusion Technique for Electronic Halftoning" by Billotet-Hoffmann and Bryngdahl, Proc. SID vol. 24/3,1983, pp 252-258; and "An Adaptive Algorithm for Spatial Greyscale", by Floyd and Steinberg, Proc. SID vol. 17/2, 1976, pp 75-77.
Accordingly, a need in the art arises for an optical scanner that is capable of reproducing the scanned document without losing grey toned information, and without requiring very large amounts of data to be transferred and stored.